Bundle of fibers

Muscle fiber (muscle cell)

Myofibril Sarcomere

Myosin filamentActin filament

Relaxed

Partially contracted

Fully contracted

Z- line Z- line

M- disk

A band

H Zone

Myofibril

I band I bandA band

H ZoneZ line Z line

One Complete Sarcomere

M line

Actin TroponinTropomyosin

Thin filament organization

Myosin Tail(-helix coiled coil)

Myosin Head

Myosin light chains(LC1, LC2)

M-disk Region

ATP binding causes release of myosin from actin

ATP

Hydrolysis of ATP

ADPPi

Conformational change“cocking of myosin head”

ADPPi Formation of myosin/actin

crossbridge

“Power Stroke”

ADP

Pi

Release of ADP and Pi

High Ca2+

Ca2+

channelCa2+

pump

Ca2+/calmodulin

MLCKMLCK-Ca2+

(active)

Myosin/MLC(relaxed)

Myosin/MLC- P(contraction)

ATP ADP

Pi H2O

MLC phosphatase

MLCK- P(inactive)

MLCK phosphatase

Epinephrinereceptor

Epinephrine

G Protein

Adenylatecyclase

ATP

cAMPR2C2 Protein kinase

(inactive)

2C Protein kinase

(active)

Electrical stimulation

Monocyte membrane